Process for recovering alkali metal hydroxide from organic liquors

ABSTRACT

The specification describes a process for recovering alkali metal hydroxides from an organic liquor such as black liquors derived from pulping wood chips. The organic liquor is burned in a fluidised bed combustion furnace containing fluidised particles of an iron rich mixed oxide of an alkali metal and iron. Particles of alkali metal ferrite are extracted from the furnace and dissolved in a solution of alkali metal hydroxide to form a more concentrated solution of alkali metal hydroxide and a precipitate of the iron rich mixed oxides of alkali metal and iron. A mixed oxide disclosed in the specification generally has the following formula: NaFe 5  O 8 .4H 2  O.

FIELD OF THE INVENTION

The present invention relates to the recovery of sodium hydroxide fromorganic liquors such as black liquor.

BACKGROUND OF THE INVENTION

Wood may be mechanically or chemically pulped. The chemical pulpingprocesses normally employed are the sulphate, sulphite and soda pulpingprocesses. The economics of these processes rely heavily on the recoveryof chemicals employed in them.

In the sulphate process wood chips are digested in a solution of sodiumhydroxide, sodium sulphide and possibly some sodium carbonate. In thesulphite process wood chips are cooked in the presence of sulphurdioxide and a bisulphite or a sulphite. In the soda process the woodchips are cooked in the presence of a solution of sodium hydroxide andpossibly some sodium carbonate. The wood pulp is separated from theliquor. The liquor from the sulphate process and the soda process isnormally referred to as black liquor. The present invention is concernedwith the recovery of sodium hydroxide from black liquors derived fromthe soda process.

The specification of Australian Patent No. 486132 describes a method ofrecovering sodium hydroxide from black liquor which involves thefollowing process steps:

1. concentrating the black liquor;

2. mixing ferric oxide with the condensed black liquor;

3. burning the mixture to produce sodium ferrite;

4. submerging the sodium ferrite in hot water to form sodium hydroxideand a precipitate of ferric oxide;

5. reusing the ferric oxide by mixing it with more condensed blackliquor and repeating steps 3 and 4.

The specification of Australian Patent No. 519156 describes a processfor recovering sodium hydroxide from black liquor which differs from theprocess described in Patent No. 486132 by the inclusion of a coldwashing step. Sodium ferrite is washed in cold water to remove sodiumchloride, sodium sulphate and other soluble impurities. Thespecification of Australian Patent No. 519156 also describes the use ofa fluidised bed in the burning step.

The specification of Australian Patent No. 552973 describes a processsimilar to the process described in Australian Patent No. 519156 withthe exception that it includes the step of agglomerating fines of ferricoxide and fines of sodium ferrite with black liquor before or duringburning in a fluidised bed. The fines of ferric oxide are derived fromthe ferric oxide precipitated when sodium ferrite is added to warmwater.

There are also a considerable number of Japanese patent applicationsthat describe similar processes for recovering sodium hydroxide fromblack liquor. Some involve mixing the ferric oxide with the black liquorprior to combustion in a fluidised bed. Some involve combustion of theblack liquor in a fluidised bed comprised of particles of ferric oxideor other transition metal oxide such as titanium dioxide. Some alsoinvolve combustion of black liquor in a fluidised bed comprised ofanother metallic oxide as well as ferric oxide. Sodium ferrite forms onthe substrate oxide and the substrate oxide and ferric oxide arerecovered when the agglomerate from the fluidised bed is added to hotwater. The substrate oxides may be magnesia or alumina. However, onlyone patent specification describes the step of dissolving the sodiumferrite in an aqueous solution of sodium hydroxide. This is thespecification of Australian Patent No. 599933. In a preferred form ofthis process the aqueous solution of sodium hydroxide has aconcentration in the range from 10 to 150 gms per litre and morepreferably 50 to 150 gms per litre. However, again the iron oxiderecovered by the hydrolysis of the sodium ferrite is stated to be amixture of hydrated iron oxide and unreacted iron oxide.

SUMMARY OF THE INVENTION

The present applicants discovered that when sodium ferrite is added towater the temperature of the water needs to be about 80° C. before thehydrolysis of the sodium ferrite commences at an economicallysatisfactory rate. However, at this temperature the rate of hydrolysisis so rapid that the temperature of the solution becomes uncontrollableand commences to boil. Consequently, an object of the present inventionis to overcome this and other problems associated with practicing theinventions of the prior art. Accordingly, the present invention providesa method for recovering alkali metal hydroxide from an organic liquorwhich method comprises the following steps:

(i) fluidising particles of an iron rich mixed oxide of alkali metal andiron in a fluidised bed furnace;

(ii) burning in the fluidised bed furnace, an organic liquor derivedfrom treatment of organic material with an alkali metal compound, toproduce particles of alkali metal ferrite;

(iii) recovering the particles of alkali metal ferrite from thefluidised bed furnace; and cooling them to produce cooled particles ofalkali metal ferrite;

(iv) mixing the cooled particles of alkali metal ferrite with an aqueoussolution of alkali metal hydroxide at a temperature in excess of 80° C.to form a more concentrated solution of alkali metal hydroxide and aprecipitate of the iron rich mixed oxide of alkali metal and iron;

(v) recovering the more concentrated solution of alkali metal hydroxide;and

(vi) recovering the precipitate and feeding it to the fluidised bedfurnace.

Prior to step (iv) the particles of alkali metal ferrite are preferablyscreened. Oversize particles are crushed and rescreened. Undersizeparticles may be ground to a fine dust suitable for agglomeration andreturned to the fluidised bed furnace following agglomeration. Particlesof a suitable size are then used in step (iv) of the process of thepresent invention. Particles having a size in a range from 0.5 to 3 mmare suitable.

The process is particularly suited to the recovery of sodium hydroxidefrom black liquors derived from the soda process. In this case the ironrich mixed oxide compound of iron and alkali metal is an iron rich mixedoxide compound of iron and sodium. The process may also be used torecover sodium hydroxide from other organic liquors that have beenderived from treatment of an organio material with sodium compounds.

The iron rich mixed oxide compound of sodium and iron is predominantly ahydrated oxide having the following formula: NaFe₅ O₈.4H₂ O. The oxidehas a spinel like structure. The precipitation of this material in theprocess was unexpected and has a direct impact on the heat and massbalances that are a necessary step in designing a successful processplant. These balances are different to those that would arise from theteachings of the prior art.

The fluidised bed furnace may be operated at a temperature in the rangebetween 850° C. and 980° C., but preferably it is operated at atemperature in the range from 890° C. to 930° C.

The bed is preferably fluidised with air which also provides oxygen forcombusting organics contained in the black liquor. Organics in the blackliquor are combusted and sodium present in the liquor combines with theiron rich mixed oxide to form sodium ferrite. At least six phases ofsodium ferrite are known and furnace conditions are maintained such thatthe phases generated are economically leachable and do not form stickydeposits in the bed. In particular the presence of silica in haematiteor magnetite used as make-up or start-up bed material may result in theformation of a type of sodium ferrite which is not readily hydrolysed.Preferably haematite or magnetite used as start-up or make-up bedmaterial contains less than 3.5% by weight of silica. Calcium present inthe bed may result in the formation of calcium ferrite which does nothydrolyse rapidly. Materials fed to the fluidised bed should alsocontain as little sulphur as possible. Sulphur combines with iron andsodium to form low melting point materials that can causedefluidisation.

Granular sodium ferrite is removed from the fluidised bed and cooled ina cooling bed. Cooled sodium ferrite is fed to a leaching vessel toextract sodium hydroxide therefrom.

The leaching reaction rate is temperature dependant and will not occurat a reasonable rate below 80° C. but once started the reaction isstrongly exothermic and the heat release is difficult to control whencarried out in hot water. However, it has been unexpectedly discoveredthat the leaching rate is significantly slower if the ferrite isimmersed in a solution of caustic soda of concentration equal to orgreater than 150 gms per litre. In addition, a solution of caustic sodaof this or greater strength has a boiling point of approximately 108° C.or greater and thus there is a greater margin to prevent boiling than ifthe ferrite is added directly to water.

The addition of sodium ferrite to the solution of caustic soda causesthe concentration of caustic soda to increase and a precipitate of ahydrated iron rich mixed oxide of sodium and iron to form. This mixedoxide typically has the formula NaFe₅ O₈.4H₂ O. The mixed oxide isseparated from the solution of caustic soda on a belt filter. The mixedoxide is recovered from the belt and returned to the fluidised bedcombustion furnace.

A concentrated solution of caustic soda typically having a strength ofabout 250 to 300 gms per litre is recovered from the leaching vessel.The concentrated solution of caustic soda may be centrifuged to removeany suspended oxide particles and may then be treated to remove othertrace elements. The concentrated solution of caustic soda called whiteliquor is then returned to the pulp mill.

Hot flue gas from the fluidised bed combustion furnace may be passedthrough a boiler and used to generate steam for process heat and tosupply power to the plant.

Normally the flue gas is laden with a dust which is typically puresodium ferrite. The dust may be collected dry e.g. in fabric filters,agglomerated using weak black liquor as a binder and returned to thefurnace. The agglomerating process differs from other agglomeratingprocesses because the dust can chemically react exothermically with abinder and depending on the conditions, prevent agglomeration.Consequently, in order to successfully agglomerate the dust, it shouldpreferably first be cooled. In order to successfully agglomerate thedust with the liquor it is necessary to keep the temperature of themixture below 70° C. by cooling the dust and/or binder prior to mixing.The cooled dust may then be mixed with weak black-liquormicro-granulated and fed to a pelletiser.

The invention also includes a process for agglomerating dust containedin a flue gas derived from a fluidised bed furnace used for recoveringsodium hydroxide from black liquor which process comprises grinding thedust to the required particle size range, cooling the dust and/or blackliquor prior to mixing, mixing metered quantities of the ground dust andblack liquor into a microgranulator to form microgranules of the dustand agglomerating the microgranules on a pelletising pan to obtain theparticle size required for the fluidised bed furnace.

Preferably the dust is ground to a size having an even distributionabout a mean in the range from 25 to 30 micron. Dust ground to this sizeprovides pellets of optimum strength and size. The pellet size requiredpreferably lies in the range from 0.5 to 3 mm. If the proportion of bedmaterial less than 0.5 mm in size increases beyond about 20%, thefluidised bed cannot be successfully operated. The finer material isprone to stick and leads to defluidisation of the bed. This is contraryto the teaching of the specification of Australian Patent No. 519156.The fluid bed will also not accommodate more than approximately 20%oversize material as this leads to segregation and defluidisation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the process will now be described with reference to theaccompanying drawing. Black liquor from a pulp mill is fed to afluidised bed furnace (1).

Black liquor from a pulp mill is fed to a fluidised bed furnace. Thefluidised bed comprises particles of an iron rich mixed oxide of sodiumand iron. Particles of haematite or small quantities of magnetite mayalso be added as make up for iron lost in the process. The particles arefluidised by air which also provides oxygen for combustion of theorganic material contained in the black liquor. The furnace is operatedat a temperature of about 930° C. and a space velocity of about 0.2metres/second above the minimum fluidisatlon velocity using sufficientair to maintain an excess of oxygen for burning the black liquor. Sodiumferrite Granules removed from the furnace are then cooled in a coolingbed (2) to a temperature of about 160° C.

The cooled particles of sodium ferrite are screened on screens (3).Oversized pellets are sent to a crusher (4) and recycled back to thescreen (3). Undersized particles of sodium ferrite are sent to hammermills (14) where they are ground to a fine dust.

Sodium ferrite particles of a suitable size are fed to a counter flowleaching vessel (5). A solution of sodium hydroxide having aconcentration of about 100 gms per litre or higher is fed in to one endof the leaching vessel, and a solution of sodium hydroxide having aconcentration of about 300 gms per litre is withdrawn from the oppositeend of the counter flow leaching vessel. Concentrated sodium hydroxideremoved from the leaching vessel (5) is then centrifuged in centrifuges(6) to remove suspended oxide. Thereafter the concentrated solution ofcaustic soda is passed to a settling vessel (7) from which theconcentrated solution of caustic soda called white liquor is returned tothe pulp mill.

A slurry of mixed oxide and sodium hydroxide is extracted from theopposite end of the leaching vessel from which the concentrated solutionof caustic soda is extracted. The slurry is then filtered on a beltfilter (8). Filtrate recovered from the belt filter is returned to theleaching vessel (5) and the precipitate of mixed oxide is returned tothe fluidised bed furnace (1).

Dust laden flue gas from the fluidised bed combustion furnace (1) ispassed through boiler (9) to recover heat from the flue gas. Steamgenerated in boiler (9) is then used to supply process heat and powerfor the plant. The flue dust is essentially pure sodium ferrite which isrecovered in bag house (10) on fabric filters. Dust from the bag houseis mixed with dust from the hammer mill (14) and cooled in dust cooler(11). The cooled dust is then mixed with weak black liquor andmicro-granulated in a pre-mixer (12). Micro-granules from the pre-mixer(12) are fed on to a pelletising pan in the pelletiser (13) where asmall fraction of black liquor is added to agglomerate the granules intopellets and densify them. The pellets are then returned to the fluidisedbed combustion furnace (1) where they are calcined to form granulessuitable for use in the leaching circuit.

The grinding of dust to a mean size of less than 50 micron using asuitable milling process has been found to be critical to successfulpelletisation.

We claim:
 1. A method for recovering alkali metal hydroxide from organicliquors with a high degree of accuracy in mass balance, energy balance,and their transfer, which method comprises the following steps:(i)fluidising particles of a mixed oxide compound of alkali metal and ironin a fluidised bed furnace; (ii) burning an organic liquor containingorganic chemicals and an alkali metal compound in the fluidised bedfurnace, to produce particles of alkali metal ferrite; (iii) recoveringthe particles of the alkali metal ferrite from the fluidised bed furnaceand cooling them to produce cooled particles of the alkali metalferrite; (iv) mixing the cooled particles of the alkali metal ferritewith an aqueous solution of an alkali metal hydroxide at a temperaturein excess of 80° C., with the aqueous solution having a hydroxideconcentration sufficient to control the reaction between the alkalimetal ferrite and the aqueous solution of the alkali metal hydroxide atsaid temperature, to thereby increase the concentration of alkali metalhydroxide in the aqueous solution and to form a precipitate of the mixedoxide compound of alkali metal and iron; (v) recovering the aqueoussolution of alkali metal hydroxide; and (vi) recovering the mixed oxidecompound precipitate and feeding it to the fluidised bed furnace.
 2. Amethod according to claim 1 wherein the fluidised bed furnace isoperated at a temperature in a range from 850° C. to 980° C.
 3. A methodaccording to claim 2, wherein the fluidised bed is operated at atemperature in a range from 890° C. to 930° C.
 4. A method according toany one of the preceding claims wherein the alkali metal hydroxide issodium hydroxide and the mixed oxide compound of iron and alkali metalis a mixed oxide compound of iron and sodium.
 5. A method according toclaim 1 wherein the organic liquor is black liquor derived from a sodaprocess for pulping cellulosic materials wherein the cellulosicmaterials are cooked in a solution comprising sodium hydroxide.
 6. Amethod according to claim 4, wherein the mixed oxide compound of sodiumand iron is NaFe₅ O₈.4H₂ O.
 7. A process according to claim 1 whereinthe more solution of alkali metal hydroxide resulting from step (iv) hasa concentration in a range from 250 to 300 grams/litre.
 8. A processaccording to claim 1 wherein the cooled particles of alkali metalferrite are screened prior to step (iv) to provide particles of alkalimetal ferrite of a size suitable for step (iv).
 9. A process accordingto claim 1 wherein the aqueous solution of alkali metal hydroxide instep (iv) has an alkali metal hydroxide concentration equal to orgreater than 150 gms per litre.
 10. A process according to claim 5wherein the solution for cooking the cellulosic materials furthercomprises sodium carbonate.